Let's analyze the text:
Unmatched in both his skill with unique armaments and his biting sarcasm, Jax is the last known weapons master of Icathia.
This one is clear, he is the last weapon master of Icathia, at least the last that we know of.
After his homeland was laid low by its own hubris in unleashing the Void,
Ichathia was destroyed by the Void
Jax and his kind vowed to protect what little remained.
This one is the complicated one... But if we consider the other paragraphs i can guess that: Jax is from a race of creatures that lived specifically in Icathia, he is the last weapon master of his race, but there are other members of his race that survived the attack of the Void and they are protecting the ruins of Icathia? So, he is not a human mutated by the void or a troll or a darkin or a yordle or even a creature from the void or any race that has been mentioned in the lore so far.
As magic now rises in the world, this slumbering threat stirs once more, and Jax roams Valoran, wielding the last light of Icathia and testing all warriors he meets to see if any are strong enough to stand beside him.
And this is that he is doing now.
